42 ME. A. HARKER ON THE GEOLOGICAL [Feb. I906, 



the basalts, but also among the subjacent Jurassic strata along the 

 eastern coast of Skye and in the northern part of Eigg. Here their 

 intrusive nature has long been recognized. It seems an arbitrary 

 interpretation to make the dolerites intrusive when they occur 

 below the base of the basalt-group, and extrusive when they 

 occur above that line. 1 



More striking evidence is afforded by the volcanic agglomerates 

 which are found in the basalt-group, mostly near its base but also 

 at higher horizons. The fragments in these agglomerates are of 

 basalt, sometimes with an admixture of sandstones and other rocks 

 of extraneous origin ; but I have sought in vain for any undoubted 

 fragment of dolerite similar to that forming the strong sheets in 

 the neighbourhood. The same remark applies to the fluviatile 

 conglomerates intercalated at various horizons in the basalt-group 

 in Skye, Canna, Rum, and Mull. Since the dolerites play at least 

 an equal part with the basalts in the succession as now seen, and 

 are moreover of much more durable nature, the absence of frag- 

 ments of the dolerites from these accumulations points to the 

 conclusion that they were not there when the latter were formed ; 

 or, in other words, that they are intrusive sills of later date. 



Equally noteworthy is the horizontal distribution of the dolerite- 

 sheets, as brought out by the mapping of Skye. The plutonic 

 intrusions — peridotites, gabbros, and granites — which make the 

 central mountain-tract of this island are surrounded by a belt 

 of metamorphism, in which the basaltic lavas have a hard and 

 tough consistency, with an absence of bedding, markedly in 

 contrast with their usual characters. As the dolerite-sheets are 

 traced from the plateau-country towards the mountains, they 

 are found to die out, and they all terminate without entering the 

 belt of hard massive basalt of the mountain-border. Further, in 

 the mountains themselves, the gabbro and granite enclose many 

 patches of basalt in a more or less metamorphosed state, some as 

 much as a mile in length ; but the dolerite-sheets never form part 

 of these enclosed patches. 



Having regard to all these facts, the proof seems to be com- 

 plete, not only that the dolerites are intrusive in the basalts, but 

 that there is a great difference in age between the two groups, the 

 basalts being older than the plutonic intrusions and the dolerites 

 younger. Many other facts might be cited as confirming, directly 

 or indirectly, these conclusions. The general sequence of the 

 Tertiary igneous rocks established in Skye is found to be borne out 

 in the other islands, and in particular the recognition of three suc- 

 cessive phases of igneous activity in this region. These three phases 

 are characterized respectively by (i) volcanic extrusions, (ii) plutonic 

 intrusions, (iii) minor intrusions of hypabyssal habit ; and the first 

 event of the third phase is represented by the great group of 

 dolerite-sills. 



1 Sir Archibald Geikie long ago recognized the intercalation of intrusive sills 

 among the basalts, but he allowed them no importance except at the base of 

 the series. 



